A temperature control method for a polyethylene film blowing process

Abstract

The application discloses a temperature control method for a polyethylene film blowing process, which comprises the following steps: dividing an extruder into N temperature control zones along an axial direction and independently controlling the temperature of the zones, dividing a temperature control process into three stages of preheating, steady production and shutdown, heating the zones at different heating rates in the preheating stage, and the heating rate of a corresponding area of a melting section is lower than that of a corresponding area of a feeding section, starting the extruder in the steady production stage, maintaining a reference temperature of each temperature control zone, dynamically compensating an actual temperature setting value according to a material residence time, collecting an actual temperature of a melt at an outlet of a die, and if a deviation of the actual temperature from a setting value of a corresponding temperature control zone of the die exceeds a threshold value, adjusting the setting value in a reverse direction from a corresponding area of the die section to a corresponding area of the feeding section. In the shutdown stage, each temperature control zone is cooled to a second target temperature at a respective cooling rate in the order from the corresponding area of the die section to the corresponding area of the feeding section. The method overcomes the defects of time and space mismatch of the existing static temperature control, and realizes accurate matching of a temperature field and material demand.

Description

Technical Field

[0001] This invention relates to the field of polyethylene film manufacturing technology, and in particular to a temperature control method for polyethylene film blowing process. Background Technology

[0002] In polyethylene film blowing technology, temperature is a key process parameter affecting plasticization quality, film appearance, and mechanical properties. Extruders are typically divided axially into multiple temperature-controlled zones, including a feeding zone, melting zone, homogenization zone, and die section, each with independent heating and temperature sensing elements. Based on this, existing technologies have developed various temperature control schemes: for example, using PID algorithms for segmented and precise control of the barrel temperature to ensure uniform plastic melting; and a multi-temperature zone decoupled control system based on DSP and FPGA, which optimizes PID parameters through genetic algorithms to achieve independent temperature control in multiple zones. These schemes achieve segmented temperature settings in the spatial dimension, playing a positive role in ensuring the stability of the polyethylene blowing process.

[0003] However, existing temperature control technologies generally adopt a static setpoint control mode, where the target temperature value of each temperature zone remains basically fixed after the process parameters are set, and this constant value is maintained only through feedback adjustment during production. This control logic of "segmented temperature in space and constant temperature in time" fails to fully consider the dynamic evolution of the material state of polyethylene during extrusion, that is, the dynamic evolution from solid particles to melt and then to complete plasticization, as well as the differences in heat demand at each production stage, that is, the differences in heat demand during preheating, steady-state production, shutdown cooling, and production changeover.

[0004] It is known that if the preheating stage is directly heated at the steady-state temperature, the rapid temperature rise can easily lead to local overheating and thermal stress damage to the equipment. In steady-state production, the material residence time changes with the screw speed, and a constant temperature cannot compensate for the differences in thermal effects caused by the change in residence time. If there is a lack of a systematic cooling sequence during the shutdown stage, uneven cooling in different temperature zones can easily lead to equipment deformation and carbonization of residual materials. Therefore, the essential defect of the existing technology is that it uses a "fixed value" to deal with a "dynamic process," resulting in a spatiotemporal mismatch between the temperature field and the actual needs of the material—either overheating at a certain stage, leading to decomposition, yellowing, and brittle film, or undercooling at a certain stage, leading to poor plasticization, crystal points, clouding, and easy breakage. Summary of the Invention

[0005] Therefore, it is necessary to provide a temperature control method for polyethylene film blowing process to address the technical problem of spatiotemporal mismatch between temperature field and material requirements in the existing technology.

[0006] A temperature control method for a polyethylene film blowing process includes the following steps:

[0007] Step 1, Spatial Partitioning: Divide the extruder axially from the feeding end to the die end into N temperature control zones, where N≥5. Each temperature control zone can be independently controlled, and each temperature control zone covers a segment of the axial interval corresponding to the feeding section, melting section, homogenization section, and die section.

[0008] Step 2: Time Segmentation and Parameter Determination: Based on the functional segment information corresponding to each temperature control zone, the temperature control process is divided into a preheating and heating stage. Steady-state production stage and shutdown cooling phase And determine the target temperature and rate of change for each temperature control zone at each stage;

[0009] Step 3, Preheating and Temperature Rise: In In each stage, the temperature control zone heats up to its own first target temperature at different heating rates. The heating rate of the corresponding area of ​​the melting section is lower than that of the corresponding area of ​​the feeding section, thus achieving the preheating completion state.

[0010] Step 4, Steady-state production steps: Following the completion of preheating, start the extruder and proceed... During this phase, each temperature control zone maintains its own reference temperature; the actual temperature setpoint T of the i-th temperature control zone... i_set Dynamic compensation is performed based on the residence time of materials within the temperature-controlled zone: T i_set =T i_base ×α i , where α i This is a compensation coefficient, inversely proportional to the dwell time;

[0011] Step 5, Closed-loop correction steps: In During the phase, the actual temperature of the melt at the die outlet is collected. If the deviation between the actual temperature and the current set value of the temperature control zone corresponding to the die exceeds the threshold, the temperature set value of each temperature control zone is adjusted in reverse from the corresponding area of ​​the die section to the corresponding area of ​​the feeding section.

[0012] Step Six: Shutdown and Cooling: Following the steady-state production stage, the following steps are performed sequentially: stopping feeding, emptying residual materials, and stopping the extruder. In each stage, following the sequence from the corresponding area of ​​the die head section to the corresponding area of ​​the feeding section, each temperature control zone cools down to its respective second target temperature at its own cooling rate.

[0013] Step 7, Temperature Difference Constraint Step: At least in and During the phase, the temperature difference between adjacent temperature control zones is monitored in real time. If the temperature difference exceeds a preset threshold, the rate of change of the temperature control zone with the higher temperature is adjusted so that the temperature difference does not exceed the threshold.

[0014] In one embodiment, in step one, the N temperature control zones are each independently equipped with a temperature detection element and a heating execution unit, and the axial intervals covered by the feeding section, the melting section, the homogenizing section and the die head section each contain at least one temperature control zone.

[0015] In one embodiment, in step three, the heating rate is specifically: the temperature control zone within the melting section is ≤2℃ / min, and the temperature control zone within the feeding section is ≤5℃ / min.

[0016] In one embodiment, in step four, the compensation coefficient α of the i-th temperature control zone... i =k×(τ _ref / τ i ), where τ i τ represents the actual residence time of the material in the i-th temperature control zone. _ref The reference dwell time is given by k, which is the calibration coefficient.

[0017] In one embodiment, the actual residence time τ of the material in the i-th temperature control zone is... i =L i / v i L i v is the axial length of the i-th temperature control zone. i v is the average propulsion velocity of the material in the i-th temperature control zone; i The speed is determined based on the material state: the feeding section is based on the solid conveying speed, the melting section is based on the speed that decreases as the melting ratio increases, and the homogenization section is based on the melt conveying speed.

[0018] In one embodiment, in step five, the threshold is ±3°C, and the acquisition period is 15 to 60 seconds; during reverse adjustment, the adjustment amount of each temperature control zone decreases axially from the die head end to the feeding end.

[0019] In one embodiment, in step six, the cooling sequence is as follows: the area corresponding to the die head section is cooled first, followed by the area corresponding to the homogenization section, then the area corresponding to the melting section, and finally the area corresponding to the feeding section; the time interval between the start of cooling in adjacent temperature control zones is determined according to the heat capacity of each temperature control zone.

[0020] In one embodiment, in step seven, the temperature difference threshold is... The stage is 30℃, in The temperature range is 20℃.

[0021] In one embodiment, Two to three minutes before the end of the phase, the heating rate of each temperature control zone decreases linearly to zero, so that the temperature curve enters... The first derivative is continuous during the phase.

[0022] In one embodiment, During this phase, infrared thermal imaging is also used to monitor the position of the condensation line of the membrane bubble. If the position of the condensation line shifts, the shift is used as an auxiliary feedback signal to coordinate the correction of the reverse adjustment in step five.

[0023] The temperature control method of the above-mentioned polyethylene film blowing process overcomes the defects of the static setting mode of temperature control in the prior art, which is "segmented in space and constant in time". It takes into account the dynamic evolution of polyethylene from solid particles to melting and then to complete plasticization during the extrusion process, as well as the differences in heat demand at different stages such as preheating, steady-state production and shutdown cooling, and realizes the spatiotemporal coupling of temperature control. Specifically: By spatial partitioning and temporal segmentation, differentiated heating rates are set for the melting and feeding sections during the preheating stage, overcoming the local overheating and thermal stress damage caused by traditional direct heating at steady-state temperatures. In the steady-state production stage, dynamic compensation for residence time is introduced, allowing the temperature setpoint to adjust in real-time according to the material's residence time. This solves the problem that a constant temperature cannot compensate for differences in thermal effects caused by variations in residence time, effectively preventing crystal points and clouding due to excessively low temperatures and material decomposition and yellowing due to excessively high temperatures. Real-time acquisition and reverse zone-by-zone adjustment of the melt temperature at the die outlet upgrades temperature control from open-loop planning to closed-loop adaptive control. During shutdown, cooling is performed sequentially from the die to the feeding end, and the temperature difference between adjacent temperature control zones is monitored, preventing equipment deformation and residual material carbonization caused by uneven cooling. This invention upgrades temperature control from "static setting" to "dynamic planning," achieving precise matching between the temperature field and the actual material requirements in both time and space, solving the technical problem of temporal and spatial mismatch between the temperature field and material requirements in existing technologies. Detailed Implementation

[0024] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention are described in detail below. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. In the description of the present invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," indicating orientation or positional relationships, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0025] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0026] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0027] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0028] It should be noted that when an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0029] This invention provides a temperature control method for a polyethylene film blowing process. The temperature control method for the polyethylene film blowing process includes seven steps: spatial partitioning, time segmentation and parameter determination, preheating, steady-state production, closed-loop correction, shutdown and cooling, and temperature difference constraint. It aims to overcome the technical problem of spatiotemporal mismatch between temperature field and material requirements in the prior art.

[0030] Step 1: Spatial Partitioning Step: Divide the extruder axially from the feeding end to the die end into N temperature control zones, where N≥5. Each temperature control zone can be independently controlled, and each temperature control zone covers a segment of the axial interval corresponding to the feeding section, melting section, homogenization section, and die section.

[0031] Specifically: In this step, the extruder has a feeding section, a melting section, a homogenizing section, and a die section in sequence along the axial direction from the feeding end to the die end. The entire axial interval covered by the feeding section, the melting section, the homogenizing section, and the die section is continuously divided into N temperature control zones along the axial direction from the feeding end to the die end, where N≥5. Each of the feeding section, the melting section, the homogenizing section, and the die section contains at least one of the N temperature control zones. Each temperature control zone is independently equipped with a temperature detection element, a heating execution unit, and a cooling execution unit. The axial position and boundary information of each temperature control zone, as well as the correspondence information between each temperature control zone and each functional segment, are obtained as partitioning information.

[0032] Step 2: Time Segmentation and Parameter Determination: Based on the functional segment information corresponding to each temperature control zone, the temperature control process is divided into a preheating and heating stage. Steady-state production stage and shutdown cooling phase And determine the target temperature and rate of change for each temperature control zone at each stage.

[0033] It should be noted that the target temperature includes a first target temperature, a reference temperature, and a second target temperature. The rate of change includes the heating rate and the cooling rate. That is to say, the target temperature and rate of change for each stage specifically include: the first target temperature and its heating rate during the preheating stage; the reference temperature and its allowable fluctuation range during the steady-state production stage; and the second target temperature and its cooling rate during the shutdown cooling stage.

[0034] The first target temperature refers to the temperature value that each temperature control zone needs to reach at the end of preheating, which is the base temperature for subsequent steady-state production. The first target temperature varies for different functional sections. The feeding section is usually set between 120-165℃ to prevent premature resin viscosity and blockage of the feed inlet; the melting section is set between 170-210℃ to reach and maintain above the viscosity temperature so that the polyethylene polymer melts sequentially; the homogenization section is set between 165-195℃ to ensure stable melt temperature and pressure; and the die section is set between 155-185℃, usually about 10℃ lower than the end of the homogenization section, to facilitate film lifting and normal film blowing. The heating rate refers to the magnitude of temperature increase per unit time when each temperature control zone rises from room temperature to the first target temperature. For example, the heating rate of the melting section is ≤2℃ / min, and the heating rate of the feeding section is ≤5℃ / min. The slower the heating rate, the smoother the heating process and the lower the thermal stress.

[0035] The reference temperature refers to the central temperature value that each temperature control zone should maintain during normal production. Its value can be the same as the first target temperature or slightly adjusted, depending on the actual production conditions. The allowable fluctuation range refers to the extent to which the actual temperature deviates from the reference temperature. In this scheme, it is set to ±2℃ to ±5℃. Fluctuations within this range will not affect product quality, while exceeding this range will trigger closed-loop correction.

[0036] The second target temperature refers to the final temperature value that each temperature control zone needs to reach after cooling is completed. It is usually set to room temperature or close to room temperature (such as 50°C) to ensure safe shutdown of the equipment. The cooling rate refers to the magnitude of temperature drop per unit time when each temperature control zone drops from the working temperature to the second target temperature. The cooling rate of the die head section is usually faster (about 5°C / min), while the cooling rate of the melting section is slower (about 2°C / min) to adapt to the heat capacity and thermal shock resistance of different temperature control zones.

[0037] Specifically: In this step, based on the partition information and the correspondence between each temperature control zone and each functional segment, as well as the axial position information of each temperature control zone, the temperature control of the polyethylene film blowing process is divided into at least three time stages, including a preheating stage. Steady-state production stage and shutdown cooling phase And determine each temperature control zone during the preheating stage. The first target temperature and heating rate, during the steady-state production stage The reference temperature and allowable fluctuation range during the shutdown cooling phase The second target temperature, cooling sequence, and cooling rate.

[0038] Step 3, Preheating and Temperature Rise: In In each stage, the temperature control zone heats up to its respective first target temperature at different heating rates. The heating rate of the melting section is lower than that of the feeding section, thus achieving the preheating completion state.

[0039] Specifically: in this step, during the preheating stage... The N temperature control zones are heated from the initial temperature to their respective first target temperature by their respective heating execution units according to their respective first target temperature and heating rate determined in step two. The heating rate of the temperature control zone located in the axial section covered by the melting section is lower than that of the temperature control zone located in the axial section covered by the feeding section. After each temperature control zone reaches its respective first target temperature, the preheating is completed.

[0040] Step 4, Steady-state production steps: Following the completion of preheating, start the extruder and proceed... During this phase, each temperature control zone maintains its own reference temperature; the actual temperature setpoint T of the i-th temperature control zone... i_set Dynamic compensation is performed based on the residence time of materials within the temperature-controlled zone: T i_set =T i_base ×α i , where α i It is a compensation coefficient, which is inversely proportional to the dwell time.

[0041] Specifically, the steady-state production step is also called the steady-state production and dynamic compensation step. In this step, following the completion of the preheating process and confirming that each temperature control zone has reached its respective first target temperature, the extruder is started to begin production, entering the steady-state production stage. The N temperature control zones maintain their temperature operation according to their respective reference temperatures and allowable fluctuation ranges determined in step two, through their respective heating and cooling execution units; where i is the temperature control zone number and i=1,2,...,N, and the temperature setpoint T of the i-th temperature control zone is... i_set Dynamic compensation is performed based on the actual residence time of the material in the i-th temperature control zone. This dynamic compensation is achieved through a temperature compensation coefficient α. i Achieve, α i =k×(τ _ref / τ i ), T i_set =T i_base ×α i In the formula τ i τ represents the actual residence time of the material in the i-th temperature control zone. _ref For reference dwell time, k is the experimental calibration coefficient, and T i_base The reference temperature for the i-th temperature control zone determined in step two.

[0042] Step 5, Closed-loop correction steps: In During the phase, the actual temperature of the melt at the die outlet is collected. If the deviation between the actual temperature and the current set value of the temperature control zone corresponding to the die exceeds the threshold, the temperature set value of each temperature control zone is adjusted in reverse from the corresponding area of ​​the die section to the corresponding area of ​​the feeding section.

[0043] Specifically: in this step, during the steady-state production phase In the process, the actual melt temperature T at the outlet of the temperature control zone within the axial range covered by the die head section is collected at a preset period Δt. _actual Calculate T _actual The current temperature setting value T of the temperature control zone located within the axial range covered by the mold head section. _setIf the deviation ΔT between them exceeds a preset threshold, the temperature setting value of each temperature control zone is adjusted in reverse order from the temperature control zone within the axial range covered by the die head section to the temperature control zone within the axial range covered by the feeding section, until |ΔT| does not exceed the preset threshold.

[0044] Step Six: Shutdown and Cooling: Following the steady-state production stage, the following steps are performed in sequence: stopping feeding, emptying residual materials, and stopping the extruder. Then, the shutdown and cooling stage begins. In this stage, following the sequence from the corresponding area of ​​the die head section to the corresponding area of ​​the feeding section, each temperature control zone cools down to its respective second target temperature at its own cooling rate.

[0045] Specifically: In this step, following the steady-state production stage, the following steps are performed sequentially: stopping feeding, emptying residual material from the extruder, and stopping the extruder. After the extruder has completely stopped and the material in the barrel has been emptied, the shutdown and cooling stage begins. The N temperature control zones are cooled to their respective second target temperatures by their respective cooling execution units according to the cooling sequence and cooling rate determined in step two. The cooling sequence is from the temperature control zone located in the axial section covered by the die head section to the temperature control zone located in the axial section covered by the feeding section.

[0046] Step 7, Temperature Difference Constraint Step: At least in and During the phase, the temperature difference between adjacent temperature control zones is monitored in real time. If the temperature difference exceeds a preset threshold, the rate of change of the temperature control zone with the higher temperature is adjusted so that the temperature difference does not exceed the threshold.

[0047] Specifically: in this step, during the preheating stage... and the shutdown and cooling phase In the process, the temperature difference between adjacent temperature control zones is monitored in real time. If the temperature difference between adjacent temperature control zones exceeds the preset temperature difference threshold, the heating rate or cooling rate of the temperature control zone with the higher temperature is actively adjusted so that the temperature difference between adjacent temperature control zones does not exceed the preset temperature difference threshold.

[0048] In one embodiment, in step three, the heating rate of the melting section is ≤2℃ / min, and the heating rate of the feeding section is ≤5℃ / min. This is understood to be a selection of a differentiated heating rate control scheme based on polyethylene melting theory and the thermodynamic characteristics of the extruder. Polyethylene is a crystalline polymer, and its melting temperature (viscous flow temperature) is typically in the range of 105–135℃. To raise the temperature from room temperature to above this range, the material needs to undergo multiple stages, including solid conveying, compaction, and melting.

[0049] In the feeding section, the material exists in the form of solid particles. It gradually heats up through friction from the screw rotation and heating from the barrel. At this stage, the material has not yet undergone a phase change, and the heat demand is mainly sensible heat, resulting in low thermal stress and the ability to withstand a relatively fast heating rate. The melting section is a critical state of solid-liquid coexistence. The material gradually transforms from solid particles into a melt, requiring the absorption of a large amount of latent heat. The molecular chains begin to move but are not yet completely unentangled. If the temperature rises too quickly, the heat cannot be evenly transferred to the interior of the material, leading to a "half-cooked" phenomenon where the surface is overheated while the interior remains solid. The temperature control zones of the extruder are physically continuous metal structures. The melting section has the highest temperature (typically 170–210℃), and the metal's thermal expansion is greatest. Rapid heating in this section generates severe thermal stress, causing uneven changes in the gap between the barrel and screw, which can severely deform or damage the equipment.

[0050] The feeding section has a lower temperature and smaller thermal expansion, resulting in a relatively lower risk of thermal stress. Polymers are poor heat conductors, and sufficient time is required for heat to transfer from the barrel wall to the material's interior. Once the material begins to melt in the melting section, the formation of the melt layer further reduces heat transfer efficiency—the melt layer acts as an insulation layer, hindering heat transfer to the internal solid core. Therefore, the melting section must employ a slower heating rate to ensure sufficient time for heat to conduct from the barrel wall to the material's interior, achieving uniform melting from the surface inwards.

[0051] Furthermore, polyethylene is a polydisperse polymer with varying molecular chain lengths, and different molecular weight segments have different melting temperatures. Excessive heating will cause low molecular weight components to melt first while high molecular weight components remain unaware, resulting in an uneven melt. Slow heating, on the other hand, allows the polyethylene polymers of varying chain lengths to melt sequentially, achieving uniform plasticization.

[0052] Therefore, a slow heating rate of ≤2℃ / min in the melting section allows sufficient time for uniform heat transfer, preventing localized overheating. Excessively high temperatures can cause polyethylene to undergo molecular chain breakage, cross-linking, or oxidative degradation, leading to yellowing, brittleness, and decreased mechanical properties in the film. Slow heating ensures a uniform temperature field throughout the melting process, mitigating the risk of overheating and decomposition from the outset. This prevents the material from overheating and decomposing.

[0053] The feeding section's heating rate is ≤5℃ / min (relatively fast), which can quickly preheat the material to near the viscous flow temperature, improving production efficiency; the melting section's rate is ≤2℃ / min, ensuring that the material has sufficient time to absorb the latent heat of molten metal, allowing the molecular chains to fully untangle and completely plasticize. This differentiated strategy takes into account both efficiency and fundamentally avoids problems such as crystal points, clouding, and poor plasticization caused by excessively low temperatures or insufficient time.

[0054] Finally, slow heating allows the thermal expansion of the molten metal structure to be released smoothly, avoiding thermal shock and thermal stress concentration, effectively preventing barrel deformation, screw wear and premature aging of heating coils, and extending the service life of the equipment.

[0055] In summary, the heating rate limit is not set arbitrarily, but is a reasonable constraint based on the polyethylene melting theory and the thermodynamic characteristics of the extruder—slow heating in the melting section to maintain plasticization and prevent decomposition, and rapid heating in the feeding section to improve efficiency. The two work together to achieve spatiotemporal coupling control of "temperature-time-position".

[0056] In one embodiment, in step four, the preset range is ±2℃ to ±5℃. It should be noted that extruder temperature control is a typical time-varying, hysteretic, and nonlinear system. While heat is conducted to the material through the barrel wall, the shear friction generated by the screw rotation also continuously generates heat. The superposition of these two heat sources means that the actual temperature cannot remain absolutely constant at a certain point and will inevitably fluctuate around the set value.

[0057] Therefore, ±2℃ represents the lower limit of control accuracy that a conventional industrial temperature control system can stably maintain. Below this value, frequent switching of heating and cooling actuators will exacerbate system oscillations. ±5℃ corresponds to the critical offset of the polyethylene melt processing window. When the temperature rises to 5℃ above the set value, the polyethylene molecular chains begin to show obvious signs of increased thermal motion and even thermal degradation. When the temperature drops to 5℃ below the set value, the melt viscosity increases significantly, and the plasticizing quality begins to deteriorate. This range represents the optimal engineering match between the temperature control system's capability and the material's temperature tolerance.

[0058] However, allowing for slight fluctuations rather than pursuing absolute constant temperature allows the temperature control system to obtain a reasonable dead zone, avoiding actuator wear and temperature oscillation divergence caused by frequent alternation between heating and cooling, significantly improving the long-term operational stability of the system and the service life of the equipment. Simultaneously, the fluctuation range is strictly limited to a range where the quality of the polyethylene melt does not undergo qualitative changes. Excessive temperature will not cause resin decomposition, film yellowing, or brittleness, while excessively low temperature will not cause poor plasticization, crystal points, or clouding, thus minimizing the negative impact of temperature deviations on film thickness uniformity and surface gloss. This fluctuation range dynamically connects with the closed-loop correction threshold (±3℃) in step five; that is, when the fluctuation exceeds this range, reverse zone-by-zone compensation is triggered, giving the entire control strategy both planning and adaptive capabilities.

[0059] In one embodiment, in step four, the actual residence time τ of the material in the i-th temperature zone is... i Determine τ in the following manner: i =L i / v i L i v is the axial length of the i-th temperature zone. iThe average propulsion velocity of the material in the i-th temperature zone; the average propulsion velocity v i The speed is determined based on the material state in different sections: in the feeding section, it is determined by the solid conveying speed; in the melting section, it is determined by the speed that decreases as the melting ratio increases; and in the homogenization section, it is determined by the melt conveying speed.

[0060] In one embodiment, in step five, the preset period Δt is 15 to 60 seconds, and the preset threshold is ±3°C. The preset period Δt of 15 to 60 seconds is set based on the thermal inertia and hysteresis characteristics of the extruder temperature control system. Extruder heating is a typical example of a large-hysteresis, nonlinear temperature control object—1, temperature changes have significant hysteresis—after the heating actuator is powered on, heat needs to be conducted to the material through the barrel wall, resulting in a significant delay in response time. If the sampling period is too short (e.g., less than 15 seconds), frequent adjustments before the temperature has been fully transferred will cause the control system to overreact to subsequent changes that have not yet manifested, leading to frequent actuator movements and temperature oscillations; if the sampling period is too long (e.g., more than 60 seconds), it may miss the trend of temperature deviation and delay the correction opportunity. The range of 15-60 seconds precisely covers the thermal response time constant of most extruder temperature control systems, ensuring that the control command matches the actual system response.

[0061] The preset threshold of ±3℃ is determined based on the sensitivity of polyethylene melt quality to temperature deviation. That is, polyethylene has a wide melting temperature window. When the temperature fluctuates within the set value of ±3℃, the rheological properties of the melt will not change significantly. Once this range is exceeded, the processing temperature of low-density polyethylene (LDPE) exceeding the range of 160-170℃ may lead to material decomposition or poor plasticization.

[0062] Thus, the 15-60 second sampling period matches the thermal response characteristics of the control system with those of the extruder. This avoids misjudgments and oscillations caused by overly frequent sampling while ensuring that temperature deviation trends are captured in a timely manner, achieving a balance between system stability and control timeliness. The ±3℃ deviation threshold strictly limits temperature fluctuations to a range where the quality of the polyethylene melt remains unchanged, i.e., maintaining the status quo when the deviation does not exceed ±3℃, avoiding over-adjustment. Once the threshold is exceeded, reverse zone-by-zone adjustment is triggered, achieving precise response to temperature deviations. At the same time, it avoids frequent corrections and system oscillations caused by excessively small thresholds. This ensures that the closed-loop correction strategy maintains stable and reliable control quality under operating condition disturbances such as changes in extruder speed, ambient temperature fluctuations, and differences in raw material batches, ensuring precise and controllable temperature field during steady-state production.

[0063] In one embodiment, in step five, the adjustment amount of the temperature setting value of each temperature zone in the reverse zone adjustment decreases axially from the die head section to the feeding section, that is, the adjustment range of the die head section is the largest and the adjustment range of the feeding section is the smallest.

[0064] In one embodiment, in step six, the cooling sequence is as follows: first the die head section, then the homogenization section, then the melting section, and finally the feeding section; the time interval between the start of cooling in adjacent temperature zones is determined according to the heat capacity of each temperature zone, with the time interval between temperature zones with larger heat capacity being greater than that between temperature zones with smaller heat capacity.

[0065] In one embodiment, in step seven, the preset temperature difference threshold is set during the preheating stage. The temperature is 30°C during the steady-state production stage. The temperature is 20℃.

[0066] In one embodiment, during the preheating stage Two to three minutes before the end, the heating rate in each temperature zone decreases linearly to zero, ensuring the temperature curve enters the steady-state production stage. The first derivative is continuous over time.

[0067] In one embodiment, during the steady-state production phase In addition, an infrared thermal imager is used to scan the temperature field distribution along the height direction of the membrane bubble and monitor the position of the condensation line. If the position of the condensation line is offset, the offset is used as an auxiliary feedback signal to perform coordinated correction on the reverse zone-by-zone adjustment.

[0068] It should be noted that the temperature control method for a polyethylene film blowing process provided by this invention relates to a polyethylene film blowing equipment. The extruder of this equipment has, along the axial direction from the feeding end to the die end, a feeding section, a melting section, a homogenizing section, and a die section. The entire axial section covered by the feeding section, melting section, homogenizing section, and die section is continuously divided along the axial direction from the feeding end to the die end into N temperature control zones. N is generally 5 or higher, and in practical applications, 7 to 9 temperature control zones are preferred. For example, when N is 7, the temperature control zones from the feeding end to the die end are sequentially designated as temperature control zones 1 to 7, where temperature control zone 1 is located in the feeding section, temperature control zones 2 and 3 are located in the melting section, temperature control zones 4 and 5 are located in the homogenizing section, and temperature control zones 6 and 7 are located in the die section.

[0069] Each temperature control zone is independently equipped with a temperature sensing element, a heating actuator, and a cooling actuator. The temperature sensing element can be a K-type thermocouple or a PT100 resistance temperature detector (RTD), installed approximately 2mm from the inner wall of the barrel in each temperature control zone. The heating actuator can be a cast aluminum heating coil or a ceramic electric heating coil. The power of the heating coil in each temperature control zone is selected based on its axial length and the required heating rate; for example, a 4kW heating coil can be used in the feeding section, a 6kW heating coil in the melting section, a 5kW heating coil in the homogenization section, and a 3kW heating coil in the die head section. Each heating coil is connected to the temperature controller via a solid-state relay to adjust the heating power. The cooling actuator can be an axial flow cooling fan or a water-cooled solenoid valve. The temperature sensing element collects the actual temperature of each temperature control zone in real time and feeds it back to the temperature controller. The temperature controller drives the heating and cooling actuators according to preset temperature control logic, thereby achieving independent temperature regulation for each temperature control zone. The temperature controller can be a programmable logic controller (PLC) or an industrial computer, which pre-stores the axial position and boundary information of each temperature control zone, as well as the correspondence between each temperature control zone and each functional segment. Specifically, the temperature controller stores the sequence number, axial start coordinate, axial end coordinate, functional segment to which each temperature control zone belongs, and the corresponding temperature detection channel number, heating output channel number, and cooling output channel number. The functional segment to which it belongs refers to one of the following: feeding segment, melting segment, homogenizing segment, or die segment. This information forms the basis for subsequent temperature control zoning.

[0070] For example, taking an extruder with a length-to-diameter ratio of 30:1 as an example, nine temperature control zones can be divided axially from the feeding end to the die end. The axial length of each temperature control zone is approximately 3.3 times the screw diameter. The feeding section corresponds to temperature control zones 1 and 2, the melting section to zones 3 to 5, the homogenization section to zones 6 and 7, and the die section to zones 8 and 9. During equipment installation and commissioning, operators input the total number of temperature control zones N and the start and end coordinates of each zone into the temperature controller's parameter setting interface, based on the extruder's screw diameter, length-to-diameter ratio, and the actual layout of the heating coils and cooling fans. The temperature controller then creates a temperature control zone index table. Once the zone information is set, it is stored in the temperature controller's non-volatile memory for subsequent steps.

[0071] Upon receiving the production start command, the temperature controller reads the stored zone information for each temperature control zone and automatically retrieves the corresponding temperature parameters for each stage from the process parameter database based on the raw material grade, target output, and screw speed input by the operator. This process parameter database is pre-established through statistical optimization of a large amount of actual production data for different grades of polyethylene. It stores the target temperature and temperature change rate for each temperature control zone during the preheating, steady-state production, and shutdown cooling stages. Operators can also manually adjust these parameters on the touchscreen. The temperature controller binds these parameters to the corresponding temperature control zone index, generating a complete time stage and temperature parameter matrix.

[0072] For example, taking the production of LLDPE film with a melt index of 2.0 g / 10 min as an example, the operator selects the corresponding raw material grade on the touch screen, inputs the target output of 150 kg / h, sets the screw speed to 60 rpm, and the parameters automatically retrieved by the temperature controller are: first target temperature of 165 degrees Celsius and heating rate of 4 degrees Celsius / min in the preheating stage feeding section, first target temperature of 195 degrees Celsius and heating rate of 1.5 degrees Celsius / min in the melting stage, and first target temperature of 185 degrees Celsius in the homogenization stage. The temperature is set at 2.5 degrees Celsius per minute, with a first target temperature of 175 degrees Celsius and a heating rate of 3 degrees Celsius per minute for the die head section. During the steady-state production stage, the reference temperatures for each temperature control zone are 160 degrees Celsius for the feeding section, 195 degrees Celsius for the melting section, 185 degrees Celsius for the homogenization section, and 175 degrees Celsius for the die head section, with an allowable fluctuation range of ±3 degrees Celsius. During the shutdown and cooling stage, the second target temperature for each temperature control zone is 50 degrees Celsius, and the cooling order is as follows: first the die head section, then the homogenization section, then the melting section, and finally the feeding section.

[0073] For example, taking the production of HDPE film with a melt index of 0.8 grams per 10 minutes as an example, the operator selects the corresponding HDPE grade, the target output is 120 kg per hour, the screw speed is 50 rpm, and the parameters set by the temperature controller are as follows: in the preheating stage, the first target temperature of the feeding section is 150 degrees Celsius, and the heating rate is 3 degrees Celsius per minute; in the melting stage, the first target temperature is 210 degrees Celsius, and the heating rate is 1.2 degrees Celsius per minute; in the homogenization stage, the first target temperature is 190 degrees Celsius, and the heating rate is 2 degrees Celsius per minute; in the die head stage, the first target temperature is 180 degrees Celsius, and the heating rate is 2.5 degrees Celsius per minute. In the steady-state production stage, the reference temperatures are 150 degrees Celsius in the feeding section, 210 degrees Celsius in the melting section, 190 degrees Celsius in the homogenization section, and 180 degrees Celsius in the die head stage, with an allowable fluctuation range of ±2 degrees Celsius. In the shutdown cooling stage, the cooling rate of each stage is reduced by about 20% compared to the LLDPE scheme to avoid internal stress caused by the rapid crystallization rate of HDPE.

[0074] During the preheating stage, the extruder is stopped, the screw does not rotate, and only the heating units of each temperature control zone are energized. The temperature controller gradually raises the temperature of each zone from room temperature according to the pre-determined target temperature and heating rate. The temperature controller reads the actual temperature of each zone in real time and compares it with the preset heating curve. When the actual temperature deviates from the preset heating curve by more than ±2 degrees Celsius, the heating output duty cycle is automatically adjusted to correct the deviation. During this process, the heating rate of all temperature control zones corresponding to the melting section is set to no more than 2 degrees Celsius per minute, and the heating rate of all temperature control zones corresponding to the feeding section is set to no more than 5 degrees Celsius per minute. Specifically, the heating rate of the melting section is lower than that of the feeding section because the melting section has the highest temperature and the greatest thermal expansion, requiring a slow heating to release thermal stress, while the feeding section has a lower temperature and less thermal stress, allowing for a relatively faster heating rate. Taking the LLDPE solution as an example, the feeding section heats the temperature from room temperature to 165 degrees Celsius at a rate of 4 degrees Celsius per minute, the melting section heats it to 195 degrees Celsius at a rate of 1.5 degrees Celsius per minute, the homogenization section heats it to 185 degrees Celsius at a rate of 2.5 degrees Celsius per minute, and the die section heats it to 175 degrees Celsius at a rate of 3 degrees Celsius per minute. Since the melting section heats up the slowest, the entire preheating cycle is determined by the melting section, taking approximately 113 minutes to reach 195 degrees Celsius from room temperature. During this process, the temperature controller records the actual temperature every 30 seconds. When the actual temperature deviates from the preset linear heating curve by more than ±2 degrees Celsius, the heating output duty cycle automatically increases or decreases by 5% to 10%.

[0075] Due to the high melt temperature of the HDPE solution, the overall heating rate of each temperature control zone during the preheating stage is relatively low: the feeding zone heats up from 3°C to 150°C per minute, the melting zone from 1.2°C to 210°C per minute, the homogenization zone from 2°C to 190°C per minute, and the die-cutting zone from 2.5°C to 180°C per minute, with a preheating cycle of approximately 154 minutes. The temperature controller's built-in watchdog timer ensures that each temperature control zone must reach its target value within the preset maximum heating time; otherwise, an alarm signal is issued.

[0076] Throughout the preheating and heating phase, the temperature controller executes a temperature difference constraint subroutine in parallel. This subroutine reads the temperature values ​​of all adjacent temperature control zones in real time at a 1-second interval and calculates the temperature difference between them. If the temperature difference between adjacent temperature control zones exceeds a preset temperature difference threshold (30 degrees Celsius during the preheating and heating phase), the temperature controller performs a temperature adjustment action: reducing the heating output duty cycle of the high-temperature side temperature control zone, i.e., reducing its heating rate, slowing down the temperature rise in the high-temperature zone, and waiting for the temperature in the low-temperature zone to rise until the adjacent temperature difference falls below the threshold, then restoring the normal heating rate of the high-temperature zone. For example, in the LLDPE preheating and heating process, the feeding section rapidly heats up to 150 degrees Celsius at a rate of 4 degrees Celsius per minute, while the front section of the melting zone heats up at a rate of only 1.5 degrees Celsius per minute and reaches a temperature of only 80 degrees Celsius, resulting in a temperature difference of 70 degrees Celsius, exceeding the 30-degree Celsius threshold. Upon detecting an excessive temperature difference, the temperature controller immediately reduced the heating output duty cycle of the feeding section from 80% to 20%, causing the heating rate of the feeding section to plummet to 0.5 degrees Celsius per minute, while waiting for the temperature in the front of the melting section to continue rising. After approximately 20 minutes, the temperature in the front of the melting section reached 120 degrees Celsius, while the temperature in the feeding section was 155 degrees Celsius, a temperature difference of 35 degrees Celsius, still exceeding the limit, so the controller continued to wait. Ten minutes later, the temperature in the front of the melting section rose to 135 degrees Celsius, while the temperature in the feeding section was 158 degrees Celsius, a temperature difference of 23 degrees Celsius, falling below the threshold. The temperature controller then restored the normal heating rate of the feeding section. Thereafter, the temperature difference between the two zones remained within 30 degrees Celsius until preheating was complete.

[0077] Once all temperature-controlled zones have reached their respective first target temperatures, 2 to 3 minutes before the end of the preheating phase, the temperature controller automatically linearly reduces the heating rate of each zone to zero, ensuring the first derivative of the temperature curve remains continuous when entering the steady-state production phase. Specifically, the temperature controller calculates the preheating completion time and, starting 3 minutes prior to that time, linearly reduces the heating rate setpoint from its current value until it reaches zero at the preheating completion time. For example, if the heating rate in the feeding section is 4 degrees Celsius per minute, it linearly decreases to 0 within 3 minutes, i.e., a decrease of approximately 1.33 degrees Celsius per minute. After the heating rate reaches zero, a 5 to 10-minute temperature stabilization confirmation period is set. During this period, the temperature fluctuation range of each temperature-controlled zone must be less than ±2 degrees Celsius and last for at least 3 minutes. Once it is confirmed that each temperature-controlled zone has reached its respective first target temperature and the temperature field is stable, the temperature controller outputs a preheating completion signal.

[0078] After receiving the preheating completion signal, the temperature controller sends a start command to the extruder main motor, causing the screw to begin rotating at a preset speed. Simultaneously, it sends a feeding command to the feeder, allowing polyethylene granules to enter the extruder, and the equipment enters the steady-state production stage. During this stage, the temperature controller switches the temperature setpoint for each temperature control zone from the primary target temperature to the dynamically compensated actual temperature setpoint. The actual temperature setpoint for each temperature control zone is calculated according to formula T. i_set =T i_base ×α i Calculated and updated in real time, where T i_base α is the reference temperature for this temperature control zone. i This is the compensation coefficient. The compensation coefficient α i Through formula α i =k×τ _ref / τ i Calculate, where τ i τ represents the actual residence time of the material in the i-th temperature control zone. _ref The reference residence time is denoted by k, which is the experimental calibration coefficient. In this embodiment, the reference residence time is the average residence time of the homogenization section at the rated speed. The experimental calibration coefficient k is 1.0 for LLDPE, 0.9 for HDPE, and 1.1 for LDPE. In practical applications, the value of k is in the range of 0.8 to 1.2.

[0079] The actual residence time τ of the material i The temperature controller calculates τ in real time based on the screw speed. i =L i / v i L i v is the axial length of the i-th temperature control zone. i v represents the average propulsion velocity of the material in the i-th temperature control zone. i The value of v is determined in stages based on the material state: in the feeding stage, the material exists in the form of solid particles, v i It equals the screw speed multiplied by the screw channel depth coefficient, which is a screw geometric parameter calculated from the screw channel depth and helix angle; in the melting section, the material is in a solid-liquid coexistence state, v i The value decreases with increasing melt ratio, using the linear interpolation formula v. i =v _solid ×(1–f)+v _melt ×f, where f is the melting ratio, estimated from the melting section length and temperature distribution; in the homogenization section, the material is completely melted, v i It equals the screw speed multiplied by the melt delivery coefficient, which is determined by the melt viscosity, screw channel depth, and pressure gradient.

[0080] For example, taking LLDPE production with a screw speed of 60 rpm as an example, the temperature controller measures the actual residence time in the feeding section to be approximately 8 seconds, in the melting section approximately 15 seconds, in the homogenization section approximately 25 seconds, and in the die section approximately 12 seconds. The reference residence time is taken as the rated value of 25 seconds for the homogenization section, and the calibration coefficient k is set to 1.0. The calculated compensation coefficients are: feeding section: 1.0 x 25 x 8 = 3.125; melting section: 1.0 x 25 x 15 = 1.667; homogenization section: 1.0 x 25 x 25 = 1.0; die section: 1.0 x 25 x 12 = 2.083. To avoid over-compensation causing the temperature setpoint to exceed the equipment's capacity and safety range, the temperature controller limits the compensation coefficients: when the compensation coefficient is greater than 1.2, it is set to 1.2; when the compensation coefficient is less than 0.8, it is set to 0.8. The actual setpoint for the feeding section after the amplitude limit is 160 degrees Celsius multiplied by 1.2, which equals 192 degrees Celsius. The setpoint for the melting section is 195 degrees Celsius multiplied by 1.2, which equals 234 degrees Celsius. However, there is a safety upper limit for the melting section temperature; in this embodiment, the melting section temperature does not exceed 220 degrees Celsius, so 220 degrees Celsius is used. The setpoint for the homogenization section is 185 degrees Celsius multiplied by 1.0, which equals 185 degrees Celsius. The setpoint for the die head section is 175 degrees Celsius multiplied by 1.2, which equals 210 degrees Celsius. The safety upper limit for the die head section is 200 degrees Celsius, so 200 degrees Celsius is used. The temperature controller uses the above values ​​as the PID setpoint for each temperature control zone, and the actual heating power keeps the temperature of each temperature control zone stable near the setpoint.

[0081] It can be seen that when the screw speed increases from 60 rpm to 80 rpm, the temperature controller automatically detects the speed change and recalculates the residence time of each temperature control zone. The feeding zone is shortened to 6 seconds, the melting zone to 11 seconds, the homogenization zone to 19 seconds, and the die zone to 9 seconds. The compensation coefficient increases accordingly, but remains at 1.2 after limiting, and the actual temperature setpoint of each temperature control zone remains unchanged. However, when the speed change is large, the temperature controller can relax the upper limit of the limit to 1.3. At this time, the feeding zone is 160 x 1.3 = 208 degrees Celsius, the melting zone is 195 x 1.3 = 253.5 degrees Celsius but is limited by the safety upper limit of 220 degrees Celsius, and the die zone is 175 x 1.3 = 227.5 degrees Celsius but is limited by the safety upper limit of 200 degrees Celsius. Finally, the melting zone operates at 220 degrees Celsius, the feeding zone operates at 208 degrees Celsius, and the die zone operates at 200 degrees Celsius. The temperature controller performs the above calculations cyclically every 0.5 seconds and updates the calculation results to the PID setpoint register of each temperature control zone in real time, so that the temperature setpoint is continuously adjusted according to the screw speed.

[0082] It is worth noting that during the steady-state production phase, the temperature controller continuously performs closed-loop correction. This closed-loop correction is triggered by a preset cycle, which in this embodiment is 30 seconds. In each cycle, the temperature controller first collects the actual temperature of the melt through an infrared temperature sensor or immersion thermocouple installed at the die outlet, and simultaneously reads the current temperature setpoint of the die section temperature control zone, calculating the deviation between the actual temperature and the setpoint. If the absolute value of the deviation does not exceed a preset threshold, which in this embodiment is ±3 degrees Celsius, the current temperature setpoint of each temperature control zone remains unchanged. If the absolute value of the deviation exceeds 3 degrees Celsius, reverse zone-by-zone adjustment is triggered: starting from the temperature control zone corresponding to the die section, the temperature setpoint of each temperature control zone is adjusted sequentially towards the feeding section, with the adjustment amount decreasing axially. Specifically, the temperature setpoint of the die section temperature control zone increases or decreases by a basic step size, which in this embodiment is 2 degrees Celsius. The adjustment amount of the adjacent upstream temperature control zone is 80% of the basic step size, the next upstream is 60%, and so on until the feeding section is 20%. After adjustment, wait for the next sampling period to detect the deviation again, and repeat the above process until the absolute value of the deviation does not exceed 3 degrees Celsius.

[0083] For example, taking the case of a high die outlet temperature during steady-state LLDPE production as an example, the temperature controller detects an actual melt temperature of 182 degrees Celsius at the die outlet every 30 seconds, while the die section setpoint is 175 degrees Celsius, a deviation of +7 degrees Celsius, exceeding the +3 degree Celsius threshold. The temperature controller determines that the melt temperature is too high and initiates reverse adjustment: the die section setpoint is reduced by 2 degrees Celsius, the homogenization section by 1.6 degrees Celsius, the melting section by 1.2 degrees Celsius, the section before the melting section by 0.8 degrees Celsius, and the feeding section by 0.4 degrees Celsius. After adjustment, the setpoints of each temperature control zone shift downwards overall. After 30 seconds, the actual temperature is detected again and drops to 178 degrees Celsius, a deviation of +3 degrees Celsius, still outside the threshold. The adjustment continues for another round: the die section temperature is reduced by another 1 degree Celsius, and the upstream temperatures are reduced by 0.8, 0.6, 0.4, and 0.2 degrees Celsius respectively. The actual temperature was checked again and found to have dropped to 176 degrees Celsius, with a deviation of +1 degree Celsius, which meets the threshold requirement. The correction is now complete.

[0084] For example, in HDPE production, a sudden drop in ambient temperature leads to a low die outlet temperature. The actual die outlet temperature is 172 degrees Celsius, while the die section setting is 180 degrees Celsius, a deviation of -8 degrees Celsius, which is below the -3 degree Celsius threshold. The temperature controller initiates reverse adjustment: the die section setting is increased by 2 degrees Celsius, the homogenization section by 1.6 degrees Celsius, the melting section by 1.2 degrees Celsius, the section before the melting section by 0.8 degrees Celsius, and the feeding section by 0.4 degrees Celsius. After adjustment, the actual temperature rises to 176 degrees Celsius, with a deviation of -4 degrees Celsius, still below the threshold. Further adjustment is made: the die section setting is increased by another 1.5 degrees Celsius, and the upstream setting is increased by 1.2, 0.9, 0.6, and 0.3 degrees Celsius respectively. The actual temperature rises again to 179.5 degrees Celsius, with a deviation of -0.5 degrees Celsius, meeting the requirements, and the correction is complete. It is understandable that this closed-loop correction process runs in parallel with the aforementioned dynamic compensation based on dwell time, without interfering with each other.

[0085] In one embodiment, an infrared thermal imager can be used to assist in monitoring the temperature field of the membrane bubble during the steady-state production stage. The infrared thermal imager is installed outside the membrane bubble forming section, scanning the temperature field distribution along the height of the membrane bubble, and using image processing algorithms to identify the position of the condensation line, i.e., the boundary line where the membrane bubble transitions from a transparent molten state to a semi-transparent solid state. An upward shift of the condensation line position indicates a higher melt temperature, while a downward shift indicates a lower melt temperature. The infrared thermal imager sends the condensation line position offset in millimeters to the temperature controller. The temperature controller converts the offset into a temperature correction value, for example, 0.5 degrees Celsius for every 1 millimeter offset, which is added to the inverse adjustment of the closed-loop correction. This ensures that temperature control not only relies on the single-point temperature at the die outlet but also comprehensively considers the overall thermal state of the membrane bubble, achieving a synergistic effect of feedforward and feedback.

[0086] When the operator presses the stop button or the production schedule ends, the temperature controller initiates the shutdown and cooling process. First, pre-shutdown preparations are performed: the temperature controller sends a stop-feed command to the feeder, closing the hopper gate; simultaneously, the screw continues to rotate at its current speed, gradually discharging any remaining molten material from the barrel. The evacuation time is automatically calculated based on the screw speed and barrel volume, typically set to 3 to 5 minutes. When the temperature controller detects that the pressure inside the barrel has dropped to atmospheric pressure and there is no significant melt flow from the die outlet, it determines that evacuation is complete. Subsequently, it sends a stop command to the main motor, the screw stops rotating, and the extruder comes to a complete stop.

[0087] Then, the temperature controller enters the shutdown and cooling phase. Each temperature control zone cools down to its respective second target temperature according to the preset cooling sequence and its own cooling rate. The cooling sequence is from the die head end to the feeding end: the temperature control zone corresponding to the die head section starts cooling first, followed by the temperature control zone corresponding to the homogenization section, then the temperature control zone corresponding to the melting section, and finally the temperature control zone corresponding to the feeding section starts cooling last. The time interval between the start of cooling in adjacent temperature control zones is determined according to the heat capacity of each temperature control zone. The heat capacity is calculated by the formula C equal to mass multiplied by specific heat capacity, where the mass is the mass of the metal part of the temperature control zone, including the mass of the barrel wall and heating coil, and the specific heat capacity is the specific heat capacity of the steel. In this embodiment, the heat capacity of the die head section is the smallest, with an interval of 5 minutes; the heat capacity of the homogenization section is medium, with an interval of 10 minutes; the heat capacity of the melting section is the largest, with an interval of 15 minutes; and the heat capacity of the feeding section is medium, with an interval of 10 minutes. The cooling rate of each temperature control zone is precisely controlled by the temperature controller by adjusting the cooling intensity of the cooling execution unit, ensuring that the actual cooling rate deviates from the preset value by no more than ±1 degree Celsius per minute. When the temperature of all temperature control zones drops to their respective second target temperature (50 degrees Celsius in this embodiment), the cooling execution unit shuts down, and the cooling process ends.

[0088] For example, taking the shutdown of an LLDPE production line as an example, the die-cutting section cools from 175 degrees Celsius to 50 degrees Celsius at a rate of 5 degrees Celsius per minute, taking 25 minutes; after a 5-minute interval, the homogenization section cools from 185 degrees Celsius to 50 degrees Celsius at a rate of 3 degrees Celsius per minute, taking 45 minutes; after a 15-minute interval, the melting section cools from 195 degrees Celsius to 50 degrees Celsius at a rate of 2 degrees Celsius per minute, taking approximately 72.5 minutes; finally, the feeding section cools from 165 degrees Celsius to 50 degrees Celsius at a rate of 4 degrees Celsius per minute, taking approximately 28.75 minutes. The temperature controller monitors the actual temperature of each temperature control zone throughout the process and adjusts the cooling fan speed to ensure the actual cooling rate matches the preset value. The entire cooling process takes approximately 2.5 hours.

[0089] During the entire shutdown and cooling phase, the temperature controller also executes the temperature difference constraint subroutine in parallel. Its constraint logic is similar to that of the preheating phase, but the temperature difference threshold is different; the threshold for the shutdown and cooling phase is 20 degrees Celsius. Specifically, it monitors the temperature difference between adjacent temperature control zones in real time. If the temperature difference exceeds 20 degrees Celsius, it reduces the cooling intensity of the low-temperature side temperature control zone, i.e., reduces its cooling rate, slowing down the temperature drop in the low-temperature zone, waiting for the high-temperature zone temperature to drop until the adjacent temperature difference falls below the threshold, and then restores the normal cooling rate of the low-temperature zone. For example, during the HDPE shutdown and cooling process, the die head section starts cooling first, rapidly cooling from 180 degrees Celsius at a rate of 5 degrees Celsius per minute, while the adjacent homogenization section has not yet started cooling and its temperature remains at 190 degrees Celsius. The temperature difference of 10 degrees Celsius does not exceed the 20-degree Celsius threshold. When the die head section cooled to 120 degrees Celsius in 12 minutes, the homogenization section cooled to 180 degrees Celsius due to natural heat dissipation, resulting in a temperature difference of 60 degrees Celsius, exceeding the 20-degree Celsius threshold. Upon detecting this excessive temperature difference, the temperature controller immediately reduced the cooling fan speed of the die head section, decreasing the cooling rate from 5 degrees Celsius per minute to 1 degree Celsius per minute, while waiting for the homogenization section to cool naturally. After approximately 30 minutes, the homogenization section temperature dropped to 150 degrees Celsius, and the die head section temperature dropped to 110 degrees Celsius, still exceeding the 40-degree Celsius temperature difference. After waiting another 10 minutes, the homogenization section temperature dropped to 140 degrees Celsius, and the die head section temperature dropped to 120 degrees Celsius, meeting the 20-degree Celsius threshold. The temperature controller then resumed the normal cooling rate of the die head section. Throughout this process, the temperature difference constraint resulted in a smoother cooling curve, preventing shrinkage stress caused by excessively rapid cooling of the die head section.

[0090] The specific values ​​involved in the above embodiments, such as temperature, heating rate, cooling rate, time interval, and number of temperature control zones, can be adjusted according to actual production conditions and should not be construed as limiting the present invention. Process parameters not explicitly mentioned in the embodiments, such as screw speed and feeding rate, can be determined based on conventional technical knowledge in the art.

[0091] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A temperature control method for a polyethylene film blowing process, characterized in that, Includes the following steps: Step 1, Spatial Partitioning: Divide the extruder axially from the feeding end to the die end into N temperature control zones, where N≥5. Each temperature control zone can be independently controlled, and each temperature control zone covers a segment of the axial interval corresponding to the feeding section, melting section, homogenization section, and die section. Step 2: Time Segmentation and Parameter Determination: Based on the functional segment information corresponding to each temperature control zone, the temperature control process is divided into a preheating and heating stage. Steady-state production stage and shutdown cooling phase And determine the target temperature and rate of change for each temperature control zone at each stage; Step 3, Preheating and Temperature Rise: In In each stage, the temperature control zone heats up to its own first target temperature at different heating rates. The heating rate of the corresponding area of ​​the melting section is lower than that of the corresponding area of ​​the feeding section, thus achieving the preheating completion state. Step 4, Steady-state production steps: Following the completion of preheating, start the extruder and proceed... During this phase, each temperature control zone maintains its own reference temperature; the actual temperature setpoint T of the i-th temperature control zone... i_set Dynamic compensation is performed based on the residence time of materials within the temperature-controlled zone: T i_set =T i_base ×α i , where α i This is a compensation coefficient, inversely proportional to the dwell time; Step 5, Closed-loop correction steps: In During the phase, the actual temperature of the melt at the die outlet is collected. If the deviation between the actual temperature and the current set value of the temperature control zone corresponding to the die exceeds the threshold, the temperature set value of each temperature control zone is adjusted in reverse from the corresponding area of ​​the die section to the corresponding area of ​​the feeding section. Step Six: Shutdown and Cooling: Following the steady-state production stage, the following steps are performed sequentially: stopping feeding, emptying residual materials, and stopping the extruder. In each stage, following the sequence from the corresponding area of ​​the die head section to the corresponding area of ​​the feeding section, each temperature control zone cools down to its respective second target temperature at its own cooling rate. Step 7, Temperature Difference Constraint Step: At least in and During the phase, the temperature difference between adjacent temperature control zones is monitored in real time. If the temperature difference exceeds a preset threshold, the rate of change of the temperature control zone with the higher temperature is adjusted so that the temperature difference does not exceed the threshold.

2. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step one, each of the N temperature control zones is independently equipped with a temperature detection element and a heating execution unit, and each of the feeding section, the melting section, the homogenizing section and the die head section includes at least one temperature control zone within its respective axial range.

3. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step three, the heating rate is specifically: the temperature control zone within the melting section is ≤2℃ / min, and the temperature control zone within the feeding section is ≤5℃ / min.

4. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step four, the compensation coefficient α of the i-th temperature control zone i =k×(τ _ref / τ i ), where τ i τ represents the actual residence time of the material in the i-th temperature control zone. _ref The reference dwell time is given by k, which is the calibration coefficient.

5. The temperature control method for the polyethylene film blowing process according to claim 4, characterized in that, The actual residence time τ of the material in the i-th temperature control zone i =L i / v i L i v is the axial length of the i-th temperature control zone. i v is the average propulsion velocity of the material in the i-th temperature control zone; i The speed is determined based on the material state: the feeding section is based on the solid conveying speed, the melting section is based on the speed that decreases as the melting ratio increases, and the homogenization section is based on the melt conveying speed.

6. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step five, the threshold is ±3℃, and the acquisition period is 15 to 60 seconds; during reverse adjustment, the adjustment amount of each temperature control zone decreases axially from the die head end to the feeding end.

7. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step six, the cooling sequence is as follows: the area corresponding to the die head section is cooled first, followed by the area corresponding to the homogenization section, then the area corresponding to the melting section, and finally the area corresponding to the feeding section; the time interval between the start of cooling in adjacent temperature control zones is determined according to the heat capacity of each temperature control zone.

8. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, In step seven, the temperature difference threshold is... The stage is 30℃, in The temperature range is 20℃.

9. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, exist Two to three minutes before the end of the phase, the heating rate of each temperature control zone decreases linearly to zero, so that the temperature curve enters... The first derivative is continuous during the phase.

10. The temperature control method for the polyethylene film blowing process according to claim 1, characterized in that, exist During this phase, infrared thermal imaging is also used to monitor the position of the condensation line of the membrane bubble. If the position of the condensation line shifts, the shift is used as an auxiliary feedback signal to coordinate the correction of the reverse adjustment in step five.